With the growing use of portable electric products such as video cameras, mobile phones, portable computers, and the like, significance of secondary batteries being mainly used as their energy sources are rapidly increasing.
As opposed to a disposable primary battery, a secondary battery is rechargeable and is being studied very actively in high-tech fields, for example, digital cameras, cellular phones, laptop computer, power tools, electric bikes, electric vehicles, hybrid vehicles, high-capacity energy storage systems, and the like.
A lithium secondary battery has a high energy density per unit weight and allows quick charging, when compared to other conventional secondary batteries such as a lead storage battery, a nickel-cadmium battery, a nickel-hydrogen battery, and a nickel-zinc battery, and thus, its use is on an upward trend.
A lithium secondary battery has an operating voltage higher than or equal to 3.6V, and is used as a power source of portable electronic appliances or high output devices such as electric vehicles, hybrid vehicles, power tools, electric bikes, energy storage systems, and uninterruptible power supplies (UPS) by connecting a plurality of batteries in series or in parallel.
A lithium secondary battery has three times higher operating voltage than that of a nickel-cadmium battery or a nickel-metal hydride battery and an excellent characteristic of energy density per unit weight, and thus, is being increasingly used.
A lithium secondary battery may be classified into a lithium ion battery using a liquid electrolyte and a lithium ion polymer battery using a solid polymer electrolyte, based on a type of an electrolyte. Also, a lithium ion polymer battery may be divided into an all-solid-state lithium ion polymer battery containing no electrolyte liquid and a lithium ion polymer battery using a gel polymer electrolyte containing an electrolyte liquid, based on a type of a solid polymer electrolyte.
A lithium ion battery using a liquid electrolyte is generally used in a shape of a cylindrical or prismatic metal can used for a container that is sealed hermetically by welding. A can-shaped secondary battery using a metal can as a container has a fixed shape, which has limitations on design of an electric product using this as a power source and its volume reduction. Accordingly, a pouch-type secondary battery fabricated by putting an electrode assembly and an electrolyte into a pouch casing made from films and forming a seal has been developed and is being used.
However, a lithium secondary battery has a risk of explosion when overheated, so ensuring safety is one of the important tasks. Overheat of a lithium secondary battery occurs by various reasons, and one of them is a flow of overcurrent beyond the limit through a lithium secondary battery. When an overcurrent flows, a lithium secondary battery generates heat by Joule heating and the temperature inside the battery increases. Also, a rapid temperature increase brings about a decomposition reaction of an electrolyte solution and causes a thermal runaway phenomenon, and in the end, results in explosion of the battery. An overcurrent occurs when a rush current is applied to a battery due to insulation breakdown between a cathode and an anode caused by penetration of a pointed metal object through a lithium secondary battery or shrinkage of a separator interposed between the cathode and the anode, or due to an abnormal condition of an external charging circuit or load being connected.
Accordingly, to protect a lithium secondary battery from an abnormal situation such as occurrence of an overcurrent, the battery is used in combination with a protection circuit, and as a protection circuit, a fuse device that irreversibly disconnects a line through which a charging or discharging current flows in the event of an overcurrent is generally used.
FIG. 1 is a circuit diagram illustrating a layout and an operating mechanism of a fuse device in configuration of a protection circuit connected to a battery pack including a lithium secondary battery.
As shown in the drawing, the protection circuit includes a fuse device 1 to protect the battery pack when an overcurrent occurs, a sense resistance 2 to sense an overcurrent, a microcontroller 3 to monitor the occurrence of an overcurrent and operate the fuse device 1 when an overcurrent occurs, and a switch 4 to perform a switching operation to flow an operating current into the fuse device 1.
The fuse device 1 is installed on a main line connected to an outermost terminal of the battery pack. The main line represents a wire through which a charging or discharging current flows. In the drawing, the fuse device 1 is illustrated as being installed on a high potential line (Pack+).
The fuse device 1 is a 3-terminal element; two terminals are connected to the main line through which a charging or discharging current flows and the rest is connected to the switch 4. Also, on the inside, the fuse device 1 includes a fuse 1a which is directly connected to the main line and is melted to be ruptured at a particular temperature, and a resistor 1b which applies heat to the fuse 1a. 
The microcontroller 3 monitors whether an overcurrent is occurring or not by periodically detecting the voltage across both ends of the sense resistor 2, and when an occurrence of an overcurrent is detected, turns on the switch 4. Then, the electric current flowing through the main line is bypassed to flow toward the fuse device 1 and applied to the resistor 1b. Thus, Joule heat generated from the resistor 1b is transmitted to the fuse 1a and the temperature of the fuse 1a increases, and when the temperature of the fuse 1a reaches a rupture melting temperature, the fuse 1a is melted to rupture, as a consequence, the main line is irreversibly disconnected. When the main line is disconnected, the overcurrent does not flow any longer and the problem caused by the overcurrent may be solved.
However, a related art as above has many problems. That is, when a failure occurs in the microcontroller 3, the switch 4 is not turned on even in the situation where an overcurrent occurs. In this case, an electric current does not flow into the resistor 1b of the fuse device 1 and the fuse device 1 does not operate. Also, a separate space for disposing the fuse device 1 within the protection circuit is needed, and a program algorithm for controlling the operation of the fuse device 1 needs to be loaded in the microcontroller 3. Therefore, there are drawbacks of reduced spatial efficiency of the protection circuit and increased load of the microcontroller 3.